Endothelium-derived relaxing factor, a labile substance formed by endothelial cells, which mediates vasodilation, has been shown to be identical to nitric oxide (NO) (Moncada et al., (1988) Biochem. Pharmacol. 37, pp. 2495-2501; Furchgott et al., (1988) FASEB J. 3, pp. 2007-2018; and Ignarro, L. J. (1989) FASEB J. 3, pp. 31-36). In addition to relaxing blood vessels, NO has multiple messenger functions as has been demonstrated in macrophages (Marletta et al., (1988) Biochemistry 27, pp. 8706-8711) and in brain tissue (Garthwaite et al., (1988) Nature 336, pp. 385-388; Knowles et al., (1989) Proc. Natl. Acad. Sci. USA, 86, pp. 5159-5162; and Bredt et al., (1989) Proc. Natl. Acad. Sci. USA, 86, pp. 9030-9033). NO appears responsible for the cytotoxic effects of macrophages and neutrophils (Hibbs et al., (1987) J. Immunol., 138, pp. 550-565).
Evidence that NO mediates functions of tissues as diverse as the brain, endothelium, and blood cells suggests a wide-spread role for NO as a messenger molecule. Localizing NO formation at a cellular level throughout the body would be greatly facilitated by immunohistochemical identification of NO synthase (NOS), the NO-forming enzyme. The use of NOS to supply deficient individuals with NO-forming ability would be expedited by the purification of the enzyme. In addition, the testing of populations for a genetic abnormality leading to deficient NO formation, such as in patients with migraines, hypertension, and coronary artery disease would be hastened by the isolation of the gene encoding NOS. Thus there is a need in the art for the biological tools for the characterization and manipulation of the NO-forming enzymatic apparatus.